Update for .NET 6

Author: Shane32

QRCoderApiTests/ApiApprovalTests.cs

@@ -1,6 +1,9 @@
 using PublicApiGenerator;
 using Shouldly;
+using System;
 using System.Diagnostics;
+using System.IO;
+using System.Linq;
 using System.Reflection;
 using System.Xml.Linq;
 using Xunit;

QRCoderApiTests/QRCoderApiTests.csproj

@@ -2,7 +2,6 @@
 
   <PropertyGroup>
     <TargetFramework>net6.0</TargetFramework>
-    <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
     <IsTestProject>true</IsTestProject>
   </PropertyGroup>